Rigid non-swellable polymeric materials having neutral, hydrophilic surfaces are useful for many applications. These include chromatography supports, membranes, carriers for immobilized enzymes or immunoassay supports. Hydration of polyacrylonitrile surfaces to form acrylamide groups is well known in the art.
U.S. Pat. No. 4,110,529 (Stoy), discloses the introduction of reactive groups into the surface layer of beads during coagulation. Example 5 of the Stoy patent discloses the partial hydration of a polyacrylonitrile to 40 percent amide groups and then coagulation to form porous beads. However, beads prepared in this manner are highly swellable in water and contain a substantial amount of byproduct carboxylate groups in addition to the desired amide groups. Thus, the beads are not particularly useful as chromatographic supports. Their tendency to swell results in excessive pressure, drops and inconsistent flow rates in chromatographic columns and the presence of carboxylate groups causes non-specific binding in separation processes not involving ion exchange. Problems also arise from the high (up to 40%) amide conversion rate as high conversion to amide groups results in significant losses in bead strength and chromatographic flow due to loss of bead rigidity.
Other attempts to convert nitrile groups to amides in the prior art have involved treatment with strong acids or bases. However, both of these techniques generally lead to some formation of surface carboxyl groups. For example, U.S. Pat. No. 4,143,203 (Rigopolous) discloses solid particles possessing an impermeable rigid polyacrylonitrile core with a hydrated surface. The surface is hydrated by heating the solid polyacrylonitrile particles in a solution of sulfuric acid at temperatures ranging from 75.degree. to 95.degree. C. However, the beads formed under these conditions are non-porous and also contain a substantial amount of byproduct carboxyl groups. They are therefore not useful in non-ion exchange protein specific chromatographic applications.
The surface modification of polyacrylonitrile under basic conditions was studied by K. Ohta et al., Nippon Kaqaku Kaishi, 6. 1200 (1985) using surface infrared spectroscopy. After treating polyacrylonitrile films with 5 percent sodium hydroxide solution for 4 hours at 70.degree. C., Ohta reported finding 4.5 percent amide and 5.7 percent carboxylate groups on the surface of the film. Treatment of the film with a solution of 5 percent sodium hydroxide and 15 percent hydrogen peroxide (an aqueous alkaline peroxide reaction) for 4 hours at 70.degree. C. reportedly produced 2.1 percent amide and 0.7 percent carboxylate. These treatments are therefore not sufficiently selective.
Thus, until recently the state of the art still encountered serious drawbacks to the formation of highly selective non-swellable highly porous acrylonitrile substrates having neutral hydrophilic surfaces. The greater surface area of highly porous beads and the narrow diameter of the polymer structure, makes it critical to accurately control the extent of hydration. Conversion of more than 15 percent of the nitrile groups to amide groups results in significant losses in flow in chromatography separations. It is difficult to accurately control the extent of reaction with acidic hydration. Acidic hydration is also known to have a strong neighboring group effect which generates a "block" polymer structure. A block polymer structure at low conversion can result in non-uniform coverage of the surface. Again, this causes problems with non-specific binding in chromatography applications. A third problem with acidic hydration is the formation of carboxyl and imide groups. The presence of carboxyl groups as previously stated causes undesired ion interactions during size exclusion or affinity chromatography applications.
It has been disclosed in commonly assigned application Ser. No. 07/276,183 that alkaline peroxide hydration of nitriles, with careful control of the solvent utilized, can avoid the aforementioned problems. The reaction selectively converts nitrile groups to amide groups without side reactions to imide or carboxyl groups. By proper selection of the solvent, the reaction can be easily controlled and actually stopped at low conversion. The use of solvent, preferably methanol, allows all of the surfaces of the substrate (as hereinafter defined), to be converted. The process disclosed therein produces an even distribution of amide groups on the surface of the substrate.
The rigid nature of the polyacrylonitrile core is minimally effected by this mild treatment and thus, the substrates are non-compressible and substantially non-swellable in water. When used therein, the term "non-compressible" denoted the resistance to hydrostatic pressures in columnar beds of up to about 3000 psi. without collapsing to prevent flow therethrough.
A method has now been found to convert substrates, such as the surface treated substrates disclosed in above-discussed U.S. Ser. No. 07/276,183, such that said substrate bears pendant N-chloroamide groups on the surface thereof while the core of the substrate remains unreacted. The substrates so produced are useful as intermediates in the production of various surface treated products which bear functional moieties linked to the core of the substrate through reaction of the pendant N-chloroamide group.